Various clamping devices are currently in use in spectacle lens prescription production (‘RX workshops’), such as shown in, for example, German Standard DIN 58739-5 ‘Optical production—Clamping means for optics—Part 5: Clamping chucks for clamping block pieces in spectacle lens production’, which defines the category. A characterizing feature of the previously known clamping chucks is that they fix the block piece at a diameter, which is intended therefor, by forces acting in radial direction, i.e. in the direction of the center axis of the block piece. These forces are applied either by way of bending of a solid body joint—‘version B’ according to the Standard—or, however, in ‘classical’ manner with the help of two inclined surfaces—‘Version A’ according to the Standard: the clamping chuck is constructed as an internal cone which is slotted in the clamping region and which is supported at a closed, somewhat steeper outer cone of an annular clamping sleeve fixedly connected with the workpiece spindle—when the clamping chuck is drawn against a central clamping pin by a tension rod.
follows:
A problem with this prior art occurs in that, particularly when the block piece is made of a plastics material, it can be deformed as a consequence of the radially acting pressure forces applied by the clamping device, so that the block piece adopts a form differing from the form it normally has, i.e. without the action of such forces. This deformation of the block piece can be transmitted by way of the block material to the blocked spectacle lens blank so that, for example, the curve which is machined on the spectacle lens during surface processing can distort or twist when the block piece together with the spectacle lens is taken out of the clamping device and the spectacle lens is unblocked from the block piece, whereupon without external application of force it adopts its ‘natural’ form. This distortion/twisting of the machined, optically effective surface at the spectacle lens is regarded as critical particularly in the production of open-die surfaces, which demand very close tolerances. In addition, a very slight distortion/twisting of the curve after unblocking of the processed spectacle lens from the block piece can here remove the processed spectacle lens from the permissible tolerance range, so that it is unusable for the intended purpose, thus a reject.
In the earlier European Patent Application 08 003 335.0 of the present applicant a block piece is described which is constructed in special mode and manner in order to, inter alia, combat the above-mentioned problem. This block piece has a base body with a center axis, a workpiece fastening section at which the spectacle lens blank can be blocked by a block material, and a clamping section by way of which the block piece and thus the blocked spectacle lens blank can be held in a clamping device, wherein this block piece has the clamping section of the base body being adapted—for example by construction with three clamping projections extending in radial direction—to be clamped or held by clamping forces which are oriented substantially in a circumferential direction with respect to the center axis of the base body or substantially in a tangential direction at a spacing with respect to the center axis of the base body. As a result, clamping forces applied in circumferential direction or tangentially to the block piece cannot ‘migrate’ through the entire block piece, as in the conventional case in which the radial pressure forces are applied to the clamping section of the block piece, so that excessive deformations of the block piece are avoided, i.e. such deformations which are transmissible to the blocked spectacle lens blank and then could have the consequence of an undesired distortion/twisting of the finished workpiece geometry. A clamping device suitable for that purpose is not, however, described in the earlier European Patent Application 08 003 335.0.
What is desired is starting from the prior art as represented by, for example, German Standard DIN 58739-5, providing a device of simplest possible construction for clamping an optical workpiece, particularly spectacle lens, blocked on a block piece for processing and/or coating thereof, the device being constructed in such a manner that excessive deformation of the clamped workpiece and the accompanying above-mentioned problems are largely avoided.